Petroleum derived fuels have been the primary source of energy for over a hundred years. Petroleum, however, has formed over millions of years in nature and is not a renewable source of energy. Currently, gasoline and diesel fuels are the two major transportation fuels. Gasoline is a complex mixture of hydrocarbons and additives for improving fuel performance. The carbon number of hydrocarbons in gasoline varies from 4 to 12, with branched alkanes, cyclic alkanes and aromatics being the most abundant. Diesel fuel is a mixture of many different hydrocarbons with the carbon numbers ranging from 9 to 23, with an average of 15-16. Usually, n-alkanes and oxygenates in diesel fuel tend to increase the octane number, while branched or unsaturated hydrocarbons lower this value.
A significant amount of research in alternative fuels has been ongoing for decades. Within this field, ethanol has been studied extensively as a gasoline substitute, and the desire to use ethanol as a transportation fuel has been increasing recently (Gray et al., Curr. Opin. Chem. Biol. 10:141 (2006)). However, the efficiency of ethanol as a fuel is still in debate (Pimentel, Natural Resources Research 14:65 (2005)); Farrell et al., Science 311:506 (2006)). As such, there is great interest in developing alternative fuel molecules to ethanol, which can be produced biosynthetically, and to develop the biosynthetic pathways for enhanced production of these alternative fuel molecules using synthetic biology.
Biologically produced fuels (“biofuels”) have received considerable attention over the past few decades due to concerns over rising oil prices, impending supply constraints, and increasing global carbon dioxide emissions. In contrast to nonrenewable natural energy sources such as petroleum and coal, biofuels are derived from renewable natural sources, typically living organisms and their metabolic byproducts. As such, there is great interest in developing biofuels that can be made using reliable and reproducible methods, and that can be used in internal combustion engines such as diesel engines and jet engines. Quite surprisingly, the present invention provides such biofuels.